An ultrasound diagnostic apparatus being a kind of medical imaging diagnostic apparatus obtains image data by transmitting/receiving ultrasonic waves between an object being examined via a probe and reconstructing ultrasound images (for example, B-mode images) based on echo signals generated from the object.
In such ultrasound diagnostic apparatus, the measurement of intima media thickness (hereinafter referred to as IMT) of a blood vessel wall is implemented in order to identify arteriosclerosis or cardiovascular diseases at an early stage. A blood vessel wall is formed with a trilaminar structure consisting of tunica intima, tunica media and tunica externa in due order from the lumen side where blood flows. IMT is the sum of the thickness of the tunica intima and the tunica media, in other words the distance from the inner wall of the tunica intima to the inner wall of the tunica media.
In the ultrasound apparatus for measuring IMT, for example, the brightness distribution in the thickness direction of a blood vessel wall of image data in one line is obtained, and the local maximal point having the maximum brightness of the brightness distribution is set as tunica externa reference point A. The second local maximal point that appears from tunica externa reference point A in the lumen side is set as tunica intima reference point B. Then IMT measurement is performed by setting minimum point C that appears in the lumen side from tunica media reference point B as the inner wall of the lumen, as well as setting the midpoint between point D having the minimum brightness of the brightness distribution and tunica externa reference point A as the inner wall of the lumen (refer, for example, to Patent Document 1).    Patent Document 1: JP-A-99-318896
However, with the technique in Patent Document 1, once in a while minimum point C does not appear clearly in the lumen side from the tunica media reference point in the brightness distribution of image data. Also, setting the midpoint between point D and tunica externa reference point A as the inner wall position of the tunica externa is based on empirical rules obtained from factors such as clinical results. Due to the individual variability of objects being examined, there are cases that IMT cannot be measured accurately. Therefore there is a demand for improvement of IMT measurement.
It is the objective of the present invention to provide a medical imaging diagnostic apparatus and method with improved accuracy for the measurement of IMT.